The overall objectives of this proposal are to elucidate the role of p190-B during mammary gland development and in breast cancer progression. p190-B was identified in our laboratory in a screen to isolate genes preferentially expressed in the terminal end buds (TEBs) during mammary gland ductal morphogenesis. Both estrogen and IGF-I are necessary for TEB proliferation, and data from p190-B null mice suggests that p190-B plays a critical role in regulating the IGF-I signaling pathway via the modulation of Rho kinase and IRS-1 levels. Furthermore, defects in TEB proliferation in p190-B heterozygotes, especially within the outer cap cell layer, are very similar to those observed in IGFR-null mammary epithelial transplants. Finally, during postnatal mammary gland development, interactions between the extracellular matrix (ECM) and cell adhesion molecules, including integrins, are essential for TEB proliferation. The p190-B protein is recruited to sites of integrin clustering, where it may both enhance the intrinsic GTPase activity of Rho proteins, as well as impact on the IGF-I signal transduction pathway. Thus, p190-B may facilitate cell motility and invasion by orchestrating ECM-mediated integrin signaling. These initial studies thus support the hypothesis that p190-B is essential for both embryonic and postnatal mammary morphogenesis, probably by facilitating the invasion of TEBs into the surrounding fat pad, and that its aberrant expression may facilitate breast cancer progression. To test these hypotheses, we propose to combine mouse genetics with the in situ analysis of signal transduction pathways in models and in human tumor specimens. Specific emphasis will be placed upon studying the mechanisms by which the p190-B interacts with other signaling pathways regulated by the estrogen receptor and IGF-I, as well as with downstream effectors of the Rho pathway, to regulate TEB proliferation and migration. The aims are: 1) To analyze p190-B function in normal mammary development, using p190-B null and heterozygous mice, and Tet-regulatable transgenic mice overexpressing p190-B. 2) To elucidate molecular mechanisms by which p190-B might promote invasion and metastasis, using a Tet-regulatable p190-B-overexpressing MCF-7 cell model and MDA 231 cells where p190-B expression is knocked down using RNAi. 3) To determine the influence of p190-B on tumor formation and metastasis, using Tet-regulatable p190-B-overexpressing mice bred with a well characterized MMTV-luc-IRES-c-erbB2 transgenic mouse model. 4) To determine the correlation between p190-B expression and the evolution and metastasis of clinical breast cancer, in specimens from our extensive tumor banks.